The present invention is at least generally related to the field of detecting thermal neutrons and, more particularly, to advanced thermal neutron detectors and associated methods. Commonly owned U.S. patent application Ser. No. 15/488,382, hereinafter the '382 Application, entitled ADVANCED THERMAL NEUTRON DETECTION SYSTEM AND METHOD, which is hereby incorporated by reference, describes concerns relating to the illicit movement of nuclear materials such as, for example, plutonium and uranium, as well as prior art approaches that have been taken in attempting to detect such materials. Commonly owned U.S. application Ser. No. 15/039,842, hereinafter the '842 Application, entitled FISSILE NEUTRON DETECTOR which is hereby incorporated by reference, likewise discusses such concerns relating to illicit movement. As noted in the '382 and '842 Applications, 1.2 million kilograms of plutonium have been produced since World War II. Given the abundance of plutonium and that the key signature of plutonium is neutron emission, it remains that neutron detection is an essential component of threat detection capability.
One approach to threat detection capability resides in the use of large surface area neutron detectors. A large surface area neutron detector can be placed, for example, to the side of a road. Simulations, as well as empirical tests, have shown that a fissile neutron source in a vehicle can be detected by a detector so positioned. When the surface area of the neutron detector is large (approximately one square meter) with approximately a 10 to 15 percent intrinsic efficiency to fissile neutrons, the number of fissile neutrons measured by the neutron detector can indicate the fissile source and alert authorities to the presence of the source.
A main component of a fissile neutron detector system that can detect fissile sources is a thermal neutron detector. Fissile neutrons have an energy range of around 100 keV to 10 MeV. A thermal neutron is a neutron that has an energy of less than 0.1 eV. Thermal neutron detectors are used in combination with a moderating material to slow the fissile neutrons down to thermal energies that are then detectable by a thermal neutron detector.
FIG. 1 is a diagrammatic illustration, in elevation, of a prior art thermal neutron detector, generally indicated by the reference number 10. The detector includes a chamber 12 that is generally formed from an electrically conductive material and contains a readout gas 14 such as argon. The chamber or chamber walls can be hermetically sealed so that there is less than 1% loss of the readout gas and less than 1% ingress of atmospheric gases such as nitrogen, oxygen, and water vapor into the readout gas. Inside the chamber, an active sheet material 16 is bonded to a support structure 18. This active sheet material can be enriched lithium foil (Li-6). The support structure includes open areas that allow the active sheet material to be exposed to readout gas 14 above and below the support structure. Suitable examples serving as the support structure can be a honeycomb matrix, a wire mesh, or sheet metal where holes or apertures are cut out of the sheet metal.
The assembly comprising active sheet material 16 bonded to support structure 18 may be referred to herein as an ASM-SS (Active Sheet Material Support Structure). Within the readout gas, there is a first set of electrodes 20 above and second set of electrodes 22 below the ASM-SS. These electrodes are set to a high voltage (HV) by a power supply, including a high voltage source 30 and a series resistance 32, with respect to the ASM-SS as well as with respect to a top cathode surface 34 and a bottom cathode surface 36. Electrical interconnections 38 for the electrodes are indicated using dashed lines. The HV creates an electric field in the readout gas between the top cathode surface and the top surface of the ASM-SS, and between the bottom surface of the ASM-SS and the bottom cathode surface. Operation of thermal neutron detector 10 will be described immediately hereinafter.
Turning now to FIG. 2, a further enlarged fragmentary view of prior art detector 10 of FIG. 1 is shown. First set 20 and second set 22 of electrodes are shown as energized such that an electric field 40 is present. When a thermal neutron 44, traveling on a path indicated by a dashed line 44′ impinges onto the thermal neutron detector and enters active sheet material 16, there is a probability that the thermal neutron can be captured by the active sheet material. In the case that the active sheet material is enriched lithium metal (Li-6), the capture of the neutron by a Li-6 atom produces a resulting Li-7 atom, which then decays into two daughter particles: an alpha particle 48 (consisting of two protons and two neutrons) and a triton particle 50 (consisting of one proton and two neutrons). These two particles travel in opposite directions, as shown, and lose energy as they travel through the Li-6. In the case that either the alpha particle or the triton enters the readout gas and still has sufficient kinetic energy (for example, the alpha or the triton has approximately 10% or more of its initial kinetic energy when it enters the readout gas), the daughter particles will ionize a multitude of atoms in the readout gas and create primary electrons 54 and primary positive ions 56.
Because of electric field 40 in the readout gas created by the HV, primary electrons 54 resulting from the ionization of the multitude of atoms will drift towards the nearest electrode, and the primary positive ions created by the ionization will drift towards the ASM-SS or the top or bottom cathode surfaces, depending on which is closer to the primary ion. Once the primary electrons reach a given distance from an electrode, which defines the so called Townsend avalanche region, the electric field is strong enough to accelerate the primary electrons fast enough that they begin to strip more electrons, denominated as secondary electrons 58 from the readout gas. These stripped secondary electrons in turn accelerate and strip more secondary electrons, creating a multiplication effect of secondary electron and secondary positive ion generation. The number of secondary electrons and secondary positive ions that are formed by this multiplication is referred to as the gas gain. The movement of the secondary positive ions created in the Townsend avalanche region 60 away from the electrode creates a movement of charge within the thermal neutron detector that is amplified by a signal amplifier 62 in electrical communication with the electrodes through a decoupling capacitor 64 and measured as a neutron signal 68 in the form of a pulse.
While thermal neutron detector 10 is generally effective in detecting thermal neutrons, Applicants recognize that there is a need for improvement in view of certain aspects of the operational environment to which the thermal neutron detector may be subjected.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.